9/4
y=x^2+2mx+3m=(x+m)^2+3m-m^2所以顶点纵坐标为3m-m^2,
令t=3m-m^2=-(m-3/2)^2+9/4则t最大值为9/4,即顶点的纵坐标最大值令t=3m-m^2=-(m-3/2)^2+9/4这一步什么意思？